I've tested this on perl 5.004_04 for sun-solaris, perls 5.004_05 and 5.6 for i686-linux (redhat) and even ActiveState's 5.6.0 for Win32 and _all_ of them show the same behaviour.
What causes the difference between two variations on this bit of code is whether or not the pattern is plain text (as it says above /blah/ may be optimized to an analogue of index()). If there's no regex compilation then $& causes Segmentation faults.

Usinguse re 'debug';shows that the regex isn't re-evaluated when the $& is entered on STDIN, but it does state explicitly Omitting $` $& $' support. Must say I'm at a bit of a loss as to where the value does come from.

If I were to go out on a limb a bit I would say that I'm thinking that maybe the penalty from using $&, etc in your
code is because perl links it into plain text matches as well as compiled regexes. ie $&, etc are always
there for full compiled regex's, but index() doesn't normally return the pre-match, match and post-match strings, so the "analogue of index()" requires a bit more work to produce them.

Before waiting for the input. It actually specifies that it's omitting $&, etc support, yet when you do enter $& still gives the expected answer:

Freeing REx: `.*'
foo

If you use a plain text match (like tilly suggested with /ri/ in 'string', you don't get this result at all, as perl
doesn't handle the match in the same way, it "guesses" the result, presumably using a more index() like way of making the match:

Ada Lovelace for the palindrome
Albert Einstein for having smelly feet
Alfred Nobel for his contribution to battlefield science
Burkhard Heim for providing the missing link between science and mysticism
Claude Shannnon for riding a unicycle at night at MIT
Donald Knuth for being such a great organist
Edward Teller for being the template for Dr. Strangelove
Edwin Hubble for pretending to be a pipe-smoking English gentleman
Erwin Schrödinger for cruelty to cats
Hedy Lamarr for weaponizing pianos
Hugh Everett for immortality, especially for cats
Isaac Newton for his occult studies
Kikunae Ikeda for discovering the secrets of soy sauce
Larry Wall for his website
Louis Camille Maillard for discovering why steaks taste good
Marie Curie for the shiny stuff
Nikola Tesla for the cool cars
Paul Dirac for speaking one word per hour when socializing
Richard Feynman for his bongo skills
Robert Oppenheimer for his in-depth knowledge of the Bhagavad Gita
Rusi P Taleyarkhan for Cold Fusion
Sigmund Freud for his Ménage ā trois
Theodor W Adorno for his contribution to the reception of jazz
Wilhelm Röntgen for the foundations of body scanners
Yulii Borisovich Khariton for the Tsar Bomba
Other (please explain why)